1. Field of the Invention
The present invention relates to a dental amalgam alloy for use in dental restorations. More particularly, it relates to a process for manufacturing particles coated with silver-tin alloy for use in the preparation of dental restoration.
2. Description of the Prior Art
Conventional dental amalgam alloys currently in widespread use contain approximately 65 wt % Ag, 20 wt % Sn and 15 wt % Cu with the possible inclusion of minor amounts of other metals including gold for specific purposes. Once these alloys are amalgamated with mercury, the product takes on the appearance of small granules or flakes of whitish gray color. Normally, dental alloys are prepared by melting all or a portion of the metals which constitute the alloy together and then obtaining an ingot which is then machined or ground to a powder. Smaller amounts of other metals can then be mixed with the powdered alloy if desired, and then the resulting powdered alloy is heat treated prior to pelletizing or packaging.
While silver is primarily used as the noble metal in most dental alloys, gold can also be used. However, it is normally not used because of its significantly high cost. An example of a dental alloy containing gold, as shown in U.S. Pat. No. 1,164,997, is one in which an alloy of baser metals including tin, copper, silver, zinc, aluminum and the like is formed. After granulating or flaking the alloy, the individual particles are then provided with a coating or envelope of gold. The gold coated alloy particles can then be amalgamated and used in the normal fashion to complete a dental restoration.
Metal coated alloy or metal particles have been used for different purposes unrelated to dentistry. U.S. Pat. No. 2,294,895 shows an electrolytic powder prepared by coating a thin film of tin on copper particles. The amount of tin utilized ranges from 0.05% to 0.3%. U.S. Pat. No. 2,273,832 shows a similar type of alloy powder in which copper particles are coated with tin, while U.S. Pat. No. 2,286,237 shows the coating of copper particles with such metals as zinc, lead, cadmium and bismuth. U.S. Pat. Nos. 2,018,343 and 3,202,488 show the preparation of electrically conductive metal particles in which particles of a base metal such as copper or zinc are coated with silver. The particles are normally applied to a substrate such as plastic thereby forming an electrically conductive layer on the underlying substrate.
Because of the high percentage of silver in conventional dental alloys and because of the complexity of preparing dental alloys, the cost to the dentist in the purchase of the alloy materials is very high, currently varying between $40 and $100 an ounce. At the current cost levels of silver, considerable savings potentially may be realized if a satisfactory alloy could be manufactured which contains smaller amounts of silver. British Patent Publication No. 2,051,132A discloses a silver containing dental alloy which contains substantially less silver than conventional dental alloys. The reduction in the amount of silver present in the alloy is achieved by coating a particulate core material either free of silver or containing only relatively small amounts of silver such as powdered copper with a layer of conventional silver-tin dental alloy. Since the overall alloy contains substantially less silver than conventional dental alloys, significant savings in the cost of silver can be realized. The powdered alloy is used just as a conventional dental alloy is used in the preparation of a dental amalgam restoration. However, the methods disclosed for preparing the dental alloy powder are relatively complex and are: (1 ) rumbling, (2) electroplating with an applied current, and (3) electroless plating which requires a reducing agent to supply electrons. Moreover, all of the techniques disclosed in the reference show the deposition of silver and tin as separate layers on the core particles. The reference does not show the simultaneous deposition of silver and tin in a single layer on core particles. A need, therefore, continues to exist for a simpler and cheaper method of preparing dental alloy particles comprising a single layer of silver and tin deposited on a powdered core substrate.